Communication information has been remarkably increased in line with the development of the information society. In line with an increase in information, the wavelength division multiplex (WDM) transmission has been widely employed in the field of optical transmission, and the age of wavelength division multiplex transmission has arrived. A wavelength division multiplex transmission is a system is able to transmit a plurality of optical signals each having a of different wavelength on a single optical waveguide. Such a WDM system is an optical transmission system suitable for high bit rate and high speed communications.
However, when communication is carried out with a plurality of wavelength multiplexed optical signals, a non-linear phenomenon occurs. The non-linear phenomenon brings a waveform distortion to optical signals which are transmitted, and becomes a concern in high speed and high bit rate WDM optical transmissions.
As a study in solving this non-linear phenomenon, in a report of C-176, etc. in the Autumn Convention of Electronic Information Communication Society in 1996, it is reported that this non-linear phenomenon is suppressed by limiting the non-linear refraction index n.sub.2. Furthermore, as a means for suppressing the non-linear phenomenon, it noted that the mode field diameter (MFD) of dispersion shifted optical fibers is increased. For example, Japanese laid-open patent publication No. 30106 of 1996 discloses that a mode field diameter of 10 .mu.m or more is achieved in dispersion shifted optical fibers which has a segment type refraction index distribution.